For example, a vehicle engine generally uses an electromotive force output type gas sensor which takes the exhaust gas discharged from the engine as an object of detection and detects the oxygen concentration. The gas sensor has an electrogenic cell which outputs an electromotive force signal which differs depending on whether the exhaust gas air-fuel ratio is rich or lean. Specifically, when the air-fuel ratio is rich, the gas sensor outputs an electromotive force signal of about 0.9 V and when the air-fuel ratio is lean, the gas sensor outputs an electromotive force signal of about 0 V.
As for this kind of gas sensor, attention has been drawn to the fact that when the air-fuel ratio of the exhaust gas changes to rich or lean, the sensor output changes with a delay from the actual change of the air-fuel ratio. Various techniques have been described to improve this output characteristic.
For example, in the gas sensor control device in Patent Literature 1, a constant current circuit is connected to at least one of a pair of sensor electrodes. When it is determined that a change request to change the output characteristic of the gas sensor has been generated, the direction of constant current is determined according to the change request and the constant current circuit is controlled so that the constant current flows in the determined direction. Thus, the output characteristic of the gas sensor is appropriately controlled by supplying the constant current.
On the other hand, the document describes a technique of conducting abnormality diagnosis of the gas sensor. For example, when sensor output varies, abnormality diagnosis concerning output response is conducted according to the amount of output variation (variation speed) within a given time period. However, in the above arrangement that a constant current flows in the gas sensor, for example, when the air-fuel ratio is controlled on a richer side of the stoichiometric value, sensor output is smaller than when no constant current flows (namely, the sensor output is closer to the stoichiometric value of 0.45 V) and as a consequence, the amount of output variation which can be used as a parameter for abnormality diagnosis is small. When the abnormality diagnosis parameter is small, the accuracy of abnormality diagnosis may decrease.